I have cleaned *many* CCD cover slips.
I would not recommend a dry cotton bud!!
....particularly if the cover slip is AR coated...bitter experience has taught me they can and do put sleeks on optics.
A blast of dust off, followed by gently applied and carefully formed pad of *name brand* basic white Kleenex tissue (no Aloe!) soaked with *name brand* Windex, followed by another blast of dust-off might do the trick.
To do it properly requires a filtered-air booth, conformal cleaner and microscope.